This invention generally relates to methods and apparatus for manufacturing reclosable bags. In particular, the invention relates to methods and apparatus for making reclosable packages having slider-operated string zippers.
Reclosable bags are finding ever-growing acceptance as primary packaging, particularly as packaging for foodstuffs such as cereal, fresh fruit and vegetables, snacks and the like. Such bags provide the consumer with the ability to readily store, in a closed, if not sealed, package any unused portion of the packaged product even after the package is initially opened.
Reclosable bags comprise a receptacle having a mouth with a zipper for opening and closing. In recent years, many zippers have been designed to operate with a slider mounted thereon. As the slider is moved in an opening direction, the slider causes the zipper sections it passes over to open. Conversely, as the slider is moved in a closing direction, the slider causes the zipper sections it passes over to close. Typically, a zipper for a reclosable bag includes a pair of interlockable profiled closure strips that are joined at opposite ends of the bag mouth. The profiles of interlockable plastic zipper parts can take on various configurations, e.g. interlocking rib and groove elements having so-called male and female profiles, interlocking alternating hook-shaped closure elements, etc. Reclosable bags having slider-operated zippers are generally more desirable to consumers than bags having zippers without sliders because the slider eliminates the need for the consumer to align the interlockable zipper profiles before causing those profiles to engage. In one type of slider-operated zipper assembly, the slider straddles the zipper and has a separating finger at one end that is inserted between the profiles to force them apart as the slider is moved along the zipper in an opening direction. The  other end of the slider is sufficiently narrow to force the profiles into engagement and close the zipper when the slider is moved along the zipper in a closing direction.
In the past, many interlocking closure strips were formed integrally with the bag making film, for example, by extruding the bag making film with the closure strips formed on the film. Such constructions, however, were limited by the conditions required to extrude both the film and zipper together. To avoid such limitations, many bag designs entail separate extrusion of the closure strips, which are subsequently joined to the bag making film, for example, by conduction heat sealing. These separate closure strips typically have flanges (also called fins) extending therefrom in such a way that the flanges can be joined to bag making film in order to attach the closure strips to the film. Previous slider-operated, separately extruded zippers used flange-type constructions.
An alternative zipper design is the so-called flangeless or string zipper, which has no flange portion above or below the interlockable closure profiles. In the case of a string zipper, the bag making film is joined to the backs of the bases of the closure strips. String zippers can be produced at much greater speeds, allow much greater footage to be wound on a spool, thereby requiring less set-up time, and use less material than flanged zippers, enabling a substantial reduction in the cost of manufacture and processing.
U.S. patent application Ser. No. 10/367,450, entitled “Reclosable Packaging Having Slider-Operated String Zipper”, discloses a reclosable bag in which respective marginal portions of the bag film are sealed to the backs of respective flangeless zipper strips and in which the resulting string zipper is actuated by means of a straddling-type slider having a plow that separates the zipper strips during opening.
U.S. patent application Ser. No. 10/436,433, entitled “Method and Apparatus for Inserting Sliders During Automated Manufacture of  Reclosable Bags”, disclosing a method of manufacturing bags with slider-actuated string zippers. The method involves attaching string zipper at its bases to respective portions of the bag making film using a band or “drag” sealer and then pressing a slider clip across the zippered film. The band sealer welds the zipper to the film by conductive heat sealing in approximately respective central band-shaped zones on the bases of the respective zipper strips. Excess unsealed film on the consumer side of the zipper is then removed or trimmed in a downstream process.
A razor-sharp knife can be used to trim excess film after the string zipper and film have been joined. More specifically, a pair of stationary knives can be placed on opposing sides of the zippered film to slit the excess unsealed film on both sides concurrently as the web of film is advanced on the machine. The portion of the excess unsealed film severed from the zipper-film assembly is taken away. In theory, each knife could be placed so that the knife tip is as close to the zipper as one-half of the width of the knife blade. However, the zipper wanders during the manufacturing process so that the distance between the moving zipper and the tips of the stationary knifes is subject to variation during the manufacturing process. As the zipper wanders, a knife placed too close or pressed against the zipper could cut the zipper if the zipper tended to wander towards the knife, thereby causing sufficient pressure to cut the zipper. Thus, the tip of each knife can be set as close as one-half of the width of the knife tip away, but in practice, zipper wander will result in a knife position further away from the zipper than one-half width of the knife blade to provide a tolerance for zipper wandering.
After trimming, the resulting zipper-film assembly has respective “tails” or remnants of excess unsealed film on the consumer side of the zipper-film band-shaped zones of joinder. Even if the excess film could be cut closer to the zipper-film seal, instead of one-half of the knife blade width away, invariably some tail will remain. In some cases, these tails may extend beyond the tops of the zipper strips. If these tails are not removed, several problems can result. As  the slider clips are pressed onto the zipper film, the unattached tail ends tend to gather or bunch up when the slider clip drags across the film. The gathered film can interfere with fully seating the slider clip around the zippered film. Also, the film can be pulled or peeled away from the zipper. During usage of the completed package, gathered film along the lip of the bag mouth may interfere with the function of the seated slider, which is to open and close the zipper during slider travel.
There is a need for a method and an apparatus for making slider-actuated string zipper bags having improved functionality and manufacturability.